This invention relates to an elevator control system, and more particularly to such a system for driving an induction motor for an elevator car by an emergency electric generator.
Recently, the development in control technologies are so remarkable that induction motors can be controlled as precisely as DC motors by a variable voltage, variable frequency control system. The variable voltage, variable frequency control system comprises a converter for converting an AC power to a DC power, and an inverter for inverting the DC power to an AC power having a variable voltage and a variable frequency, the last-mentioned AC power being supplied to an induction motor to drive it. Such a control system may use a pulse amplitude modulation (which is abbreviated hereinafter to a "PAM") control and/or a pulse width modulation (which is abbreviated hereinafter to a "PWM") control. The PAM control varies a DC voltage which corresponds to the peak value of the output voltage from the inverter and the PWM control regulates the mean value of the output voltage by rendering a time width thereof variable with the peak value thereof fixed.
In the elevator control system, the induction motor should be controlled over a wide speed-range from a null to the full speed thereof. When the converter delivers the full voltage during a low speed operation of the induction motor, the induction motor increases in both noise and heat generation. In order to suppress the noise and heat generation, it may be preferable to jointly use the PAM and PWM controls and to decrease DC voltage from the inverter by PAM control during the low speed operation.
There is known an elevator control system comprising the variable voltage, variable frequency control as described above selectively energized by a normal and an emergency power source to drive an induction motor for an elevator car. As described above, the DC voltage is controlled to a low magnitude when an elevator car is operated at low speeds during the acceleration and deceleration thereof. This results in a low power factor of the power source. The normal power source is generally large in capacity and therefore does not depend upon a variation in power factor thereof. The emergency power source, however, is relatively small in capacity. This means that a low power factor results in an increase in voltage drop. In order to compensate for this voltage drop, it is required to increase the capacity of the emergency power source resulting in the disadvantage that the emergency power source becomes expensive.
Accordingly it is an object of the present invention to provide a new and improved elevator control system capable of driving an elevator car by an emergency electric source small in capacity.